The present invention relates, in general, to electromagnetic acoustic transducers and inspection systems utilizing same and, in particular, to a new and useful diode expander network for electromagnetic acoustic transducers.
In many systems such as radio, sonar, and ultrasonic testing systems, it is often desirable to use the same transducer or antenna for transmitting as well as for receiving. Typically, a very sensitive receiver is used, and any noise produced by the transmitter circuitry is coupled into the front end of the receiver where it is amplified and subsequently interferes with signals being picked up by the transducer or antenna. Electromagnetic acoustic transducers (EMATs) are essentially ultrasonic devices which require high energy transmitter pulses and very sensitive, low noise receivers because of the inherent low transduction efficiency.
Many devices for "disconnecting" the transmitter from the receiver, when not transmitting, have been devised including known transmit/receive (T/R) switches. A simple and very fast acting T/R switch is formed by using what is known as a diode expander to couple the transmitter to the transducer and therefore to the receiver front end. A diode expander comprises pairs of diodes connected "back to back", that is, with the anode (in electronic schematics, the broad part of the arrowhead on each diode symbol) of one diode connected to the cathode (in electronic schematics, the point of the arrowhead connected to the short crossbar on each diode symbol) of the other diode and vice versa. This arrangement allows high voltages and currents from the transmitter to pass through the diode expander with very little loss because the maximum forward voltage drop for a diode is on the order of 1 volt. Smaller noise signals in the transmitter circuitry are effectively blocked by the diodes which act like an open circuit until the turn-on voltage of the diode is exceeded. By placing diode pairs in series, the effective blocking voltage can be increased.
However, at high frequencies the junction capacitance of the diodes can effectively conduct noise signals to the receiver front end without exceeding the turn on voltage. As used herein, the term "high frequencies" refers to radio frequency (RF) level tone burst pulses in a frequency range from approximately 100-200 KHz to approximately 5-10 MHz. The junction capacitance is a parasitic capacitance between the anode and the cathode of the diode. This capacitance allows small signals which do not exceed the diode turn-on voltage to "leak" through the diode. Another limitation at high frequencies is the switching speed of the diodes. The diode switching speed is a measure of how fast the diode can switch from its conducting state to a non-conducting state when the voltage polarity across the diode reverses. The diode must be able to switch very rapidly to be used with the high frequency AC signals typically employed in EMAT applications. Further, since typical EMAT inspection systems require high current/power levels, the diodes used in circuits for such systems must have high current/power handling capability. The current handling capability of a diode is basically how much average current the diode can conduct before it overheats. Generally, as the current/power handling capability of a diode increases, the junction capacitance increases and the switching speed decreases. This has severely limited the use of the diode expander T/R switches in known EMAT applications.
Presently, there is no known diode expander circuit which provides efficient noise blockage for EMAT applications which operate at high frequencies and with high current/power levels.